I have never covered it with anything and it held nicely the stand up fish on my Fathers day cake was RKT covered in fondant and had 0 problems with it.
The only thing though is your fondant will be really bumpy and textured so if you are looking for a smooth finish I would make sure your fondant is thick or do a thin layer wet the top of that then do the final layer so you don't see the bumpiness.
There me be other ways to do this if there are I would really like to hear them! 
HTH!
I am hand-molding the leg for the base of my cake now. I would think it would be easier than carving it. Don't make my mistake ~ wait until the RKT are completely cool before you start molding!!!
Still waiting on others to chime in about coating the RKT before covering with fondant!
Do a search for RKT and Rice Krispie Treats and that will give you a lot of info. I covered my round RKT cake with BC and let it crust and then covered with fondant. Turned out great.
I know that some people melt chocolate (white or dark) and cover it - filling in all of the spaces and covering the bumps. Some people use ganache either white or chocolate with great success - this has to set overnight. Then there are those who do a very thin layer of fondant and let that set up and then apply a thicker layer. Be careful that you don't loose the shape of your leg (I am thinking that is what you are going to be doing as I have read your other post - cool idea). I certianly would cover it with a light layer of something or you are not going to have a smooth surface.
I've tried it both ways. The first time I used 2 layers of fondant. Then I tried covering the RKT with modeling chocolate (candy clay) and then I applied the fondant. It worked great and was really smooth. I've also seen this done on Food Network Challenge and Ace of Cakes. Good luck!
Ive never used this before and I have a beer mug cake to do tomm...I was going to make the handle out of RKT's...Ive read where others have done this...but my question is this...I want to cover it in b/c icing not fondant.
Im in Texas, wont the heat keep the RKT soft? and it start falling? I want to use it for the handle and the pouring effect. But I dont want the heat to melt down the design.
I am in the process of making a book that is standing up vertically. I covered my RKT w/ white chocolate ganache, using a hot knife to smooth the ganache, and let it set up for about 12 hours. The ganache covered the RKT bumps beautifully and gave me a nice smooth surface to cover w/ fondant. I misted the book lightly w/ water before applying the fondant.
2txmedics, as long as you put the RKT in the fridge to set up, you will be fine. It will get pretty hard, and then you can ice it with BC. I ived my leg with BC, then smoothed it out, then put on the fondant. The leg looks awesome (except where my seam is ~ that was really tough!), and it hardly has any bumps in it. Hopefully, once I put the SugarVeil lace on it (for the fishnet hose) it will look even better!
Next time, I might try ganache and see how that works. As it is, my BC seeped out a little at the seams, and that makes things messy.
I cover mine with the choc. candy clay. Because of the temp. of your hands, you can rub it till it's really smooth, then I apply a bit of piping gel (to help fondant stick) and then my fondant and have found this to work really well in smoothing out the lumps.
I covered my "leg" with BC and smoothed it out. Then I applied a thick (about 1/2") layer of fondant. The seam in the back was a pain, but hopefully I will hide it with the fishnet hose. Anyway, it came out pretty good, I think! And what woman's leg doesn't have a bit of cellulite, anyway???
<M>
Quote by @%username% on %date%
%body%